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      <h1 class="entry-title">C++中逗号表达式的一个应用</h1>
    
    
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<time class='entry-date' datetime='2015-10-22T23:55:35+08:00'><span class='date'><span class='date-month'>Oct</span> <span class='date-day'>22</span><span class='date-suffix'>nd</span>, <span class='date-year'>2015</span></span> <span class='time'>11:55 pm</span></time>
        
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<div class="entry-content"><p>假设我们的程序需要调用三个函数<strong>f</strong>，<strong>g</strong>，<strong>h</strong>。其中<strong>f</strong>和<strong>h</strong>可能失败，可能成功，而<strong>g</strong>一定成功。<strong>f</strong>和<strong>h</strong>有返回值，<strong>g</strong>没有返回值。我们可能得写这样的代码：</p>

<pre><code>if (f()!=SUCCESS) {
  //abort
} else {
  g();
  if (h()!=SUCCESS) {
  }
}
</code></pre>

<p>由于<strong>g</strong>没有返回值，因此这么写似乎是天经地义的。能不能优雅点？</p>

<!--more-->

<p>借助于逗号表达式，我们可以这样：</p>

<pre><code>if (f()!=SUCCESS){
} else if (g(),false) { //逗号表达式
  //we will never be here...
} else if (h()!=SUCCESS) {
}
</code></pre>

<p>形式上更加优美，但是和之前的写法相比可读性较差，并且使用了三次比较，之前只需要两次比较。</p>

<p>在<strong>C++</strong>中，逗号表达式有时候是一种很有用的工具：(<strong>A,B,C</strong>)，则从左到右求值，最后<strong>C</strong>的（返回）值作为整个表达式的值。</p>
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